Everyone will talk about how Cameron/Miliband just edged it.
Two days later, Miliband/Cameron’s party will be up by less than the margin of error, and this will be called a “debate bounce”
Every single party will come up with a “viral” hashtag that’s based on something their leader said, like #hellyeahiwantastrongereconomynigel. No-one who isn’t a paid activist will use it.
The average person will come away with a slightly less negative view of the politician they had the most negative view of, because “he seems quite nice really”.
The average party member will come away with their view of the politician they had the most negative view of reinforced.
75% of the audience will wonder who Plaid Cymru are and if they’re there by mistake.
No-one will learn anything.
No difference will be made to the election result.
I won’t watch.

You’ll notice I used “they” when talking about generic people. However, I don’t believe the average person (as distinct from political obsessives. who you’ll note I treat separately) had formed any opinion at all, positive *or* negative, of Sturgeon, Wood, or Bennett. Sturgeon’s the only one of them who’s had any real media presence, and even there, there’s a reason why the Tories have been using Salmond rather than her in their attack ads.
On the other hand, Cameron, Miliband, Clegg, and Farage are all, for better or (in most cases) worse, very well-known figures, and all are vehemently disliked by quite large numbers of people. Thus for the supermajority of viewers, the politician they had the most negative view of would be someone who used the pronoun “he”. Hence me choosing to use it.
I only use gendered language when there’s actually a reason to do so.

I don’t know how it is in the UK, but here in the US we’re blanketed with so many negative attack ads (on both sides), that when we watch the debate we’re always a bit surprised that the guy on the other side isn’t the evil psychopath that all the attack ads say they are. I think debates are useful, and they’re often a turning point for a candidate in a competitive race (for better or worse).

We don’t have attack ads over here. There’s a ban on political parties advertising on TV at all — they’re limited to a certain number of “party election broadcasts” which have to comply with very stringent rules (and which get shown on non-commercial channels), and which only get shown a couple of times each.
We don’t know what the effect of the debates will be this time (and they’re in a very different format from last time) but in 2010 they had a weird effect — Nick Clegg did well, and so the Lib Dems got a temporary poll bounce, which had gone by the time of the actual election day. The Lib Dems increased their share of the vote slightly, but actually went down in number of seats — and this was, in part, because a lot of candidates who originally thought they had no chance of winning suddenly thought they were in with a chance, and so people campaigned in those seats rather than the target seats where we actually had a realistic chance of winning. We came second in a lot of places where we should have come third, but also in a lot of places where we should have come first, as a result.

The consensus seems to be, as Andrew says above, that Nicola Sturgeon, the leader of the SNP, won. Unfortunately, she’s not actually standing in the election, and the SNP only stands candidates in Scotland…

Even if she did “win” the debate, I doubt the SNP gained any supporters. They’re essentially a one-issue party, aren’t they? Not that there’s anything wrong with that, but everything the SNP does is geared toward what’s in Scotland’s interest. Which isn’t necessarily what’s in the *UK’s* best interest.

No, they’re not a one-issue party. They’ve actually been governing Scotland for the past few years, and have a majority in the Scottish Parliament. They have a centre-left, social democratic, vaguely liberalish (except on some matters where their biggest supporters are fundamentalist Christians and they equivocate) platform. Apparently one of the top Google search terms during the debate and for a couple of hours afterwards was “can I vote for the SNP?”, so it sounds like she may well have made a lot of supporters (although, again, many of them will have been in England, which won’t help her at all).